The proposed studies are designed to characterize the properties and behavior of a gonadotropin-responsive steroidogenesis activator peptide in the rat ovary and testis. This cycloheximide-sensitive factor (the putative "labile protein") is comparable or identical to a similar peptide, previously isolated from the rat adrenal cortex, which activates the mitochondrial side-chain cleavage of cholesterol in a cAMP-dependent manner. To achieve the objective, the following systems will be used as appropriate, dispersed, purified normal rat Leydig cells; implanted H-540 rat Leydig cell tumors; dispersed rat luteal cells; and expressed rat preovulatory granulosa cells. The sequence of the activator peptide will be determined and a radioimmunoassay for detection and quantitation will be developed. This will permit the assessment of time- and concentration-dependent changes in the intracellular content of peptide in response to gonadotropins, modulators of gonadotropin action, and inhibitors of protein translation and transcription. The testes of aging rats will also be inspected for the development of any lesions affecting this system. Details of the mechanism by which cAMP promotes a rise in the level of activator peptide will be investigated, with particular effort focused on a model invoking proteolytic generation of the functional peptide from an inactive precursor. A reticulocyte cell-free translation system and pulse-chase experiments will be employed. Additional studies are designed to address the metabolic fate of the peptide. The results of these investigations should markedly enhance our understanding of the way in which gonadotropins regulate a key reaction common to the steroidogenic pathways of their target organs, and may well have relevance for current efforts to control reproductive function in man.